The present invention relates to an air-conditioning system which is designed at least for an air-conditioning mode for cooling a thermal control medium and a heat-pump mode for heating the thermal control medium. For this purpose, the system comprises a refrigerant circuit with a compressor and an evaporator, a coolant circuit with a heat-generating unit to be cooled and with a heater for heating the thermal control medium. The system also includes a refrigerant/coolant heat exchanger which functions on the refrigerant side as a condenser or gas cooler, depending on the refrigerant used.
Air-conditioning systems of this type are employed, for example, for motor vehicles. The heat-generating unit to be cooled is conventionally an internal combustion engine used for driving the motor vehicle. An air-conditioning system of this type for a motor vehicle disclosed in DE 43 41 756 A1 contains, in addition to the condenser formed by the refrigerant/coolant heat exchanger, a further condenser which is arranged parallel to the first condenser and is designed as a refrigerant/ambient-air heat exchanger, i.e., as a refrigerant cooler. In it the refrigerant is cooled not by the coolant and not by the thermal control medium, but by ambient air. Also provided in the coolant circuit is a coolant cooler, by means of which the coolant can be cooled by ambient air. By means of various valves and bypass lines, the refrigerant can be conducted selectively through one condenser or the other. The coolant can be selectively conducted through the heater or it can bypass the heater, or it can be selectively conducted through the coolant cooler or bypass it and be conducted through the associated condenser or bypass it, or passed through it in a circulating manner. The purpose of the recirculation is to increase the contribution to the heating of the coolant in an initial operating phase of the refrigerant circuit. This is accomplished by first having only a little coolant flowing through the respective condenser, so that the coolant temperature in the condenser and therefore also the condensation temperature rise rapidly. Air to be conditioned and to be supplied to a vehicle interior can be cooled by the evaporator and heated by the heater.
Particularly in the case of so-called low-consumption vehicles, there is generally the problem that internal combustion engines with optimized consumption, for example, diesel engines with direct injection, no longer generate sufficient waste heat to heat the vehicle interior to a comfortable temperature level within a reasonable time, especially during low-load operation, for example, under city driving conditions. Even sufficient deicing of the front and side windows is often no longer guaranteed by engine waste heat alone. Another problem detrimental to air-conditioning comfort is the increasingly employed automatic start/stop devices, whereby the internal combustion engine of the motor vehicle is stopped during brief halts at traffic lights or in a traffic jam. The secondary assemblies, including the air-conditioning system compressor, also are deactivated as a result.
To cover the heating power deficit occurring for these reasons, a multiplicity of supplemental heating concepts employing a heat-pump have already been proposed, usually in the form of combined controls for refrigerant, coolant and air circuits. In a known concept, such as is disclosed, for example, in DE 198 06 654 A1, a supply-air/refrigerant heat exchanger functions as an evaporator in the air-conditioning mode to cool the supply air to the vehicle interior, and as a condenser or gas cooler in the heat-pump mode to heat the supply air. A similar concept with the selective use of a supply-air/refrigerant heat exchanger as an evaporator or condenser/gas cooler is disclosed in DE 198 13 674 C1. These known concepts require a bidirectionally operating expansion valve.
The known air-conditioning systems, in which a thermal-control medium/refrigerant heat exchanger functions as an evaporator in the air-conditioning mode and optionally as a condenser/gas cooler in the heat-pump mode, are generally faced with the problem of fogging when used in vehicles. Fogging occurs predominantly in the transitional seasons in the case of moist and cold climatic conditions. The condensed water precipitated in a reheating (drying mode) on the supply-air/refrigerant heat exchanger functioning as an evaporator is absorbed by the supply-air stream when subsequent heating operation is activated in the heat-pump mode. This is particularly the case when the supply-air/refrigerant heat exchanger functions in this operating mode as a condenser/gas cooler and the condensation water adhering to it evaporates into the supply-air stream. In this situation, it may lead to fogging of the vehicle window onto which the supply-air stream flows.
The known air-conditioning system concepts (involving the direct release of condensation heat or hot-gas heat in a supply-air/refrigerant heat exchanger functioning as an evaporator in the air-conditioning mode) also present a problem during the heat-pump mode. A large temperature gradient may be formed over the supply-air side outlet surface of this heat exchanger and cause significant temperature differences at various air outlet nozzles, to which the supply-air stream is apportioned in order to be fed into various vehicle interior regions.
One object of the present invention is to provide an improved air-conditioning system of the type initially mentioned, which by relatively simple means allows effective air-conditioning operation, on the one hand, and heat-pump operation, on the other hand.
A particular object is to provide such a system that avoids overhumidifying the thermal control medium during a changeover from the air-conditioning mode or a reheating mode to the heat-pump mode, as well as the disturbing fogging resulting therefrom in vehicle applications.
Another object is to provide an air-conditioning system with the capacity for comfortable heating of the supply air in vehicle applications in the heat-pump mode, without pronounced temperature differences at various air outlet nozzles.
Still another object of the invention is to provide a motor vehicle having an improved air-conditioning system.
It is also an object of the invention to provide an improved method of conditioning air that is supplied to the passenger compartment of a motor vehicle.
In accomplishing these and other objects, there has been provided in accordance with one aspect of the present invention an air-conditioning system which is suitable for a motor vehicle and which is capable of operating at least in an air-conditioning mode for cooling a thermal control medium, preferably air, and in a heat-pump mode for heating the thermal control medium. The system comprises a refrigerant circuit comprising a compressor and an evaporator, wherein the evaporator includes a second refrigerant/coolant heat exchanger; a coolant circuit comprising a heat-generating unit to be cooled and a heater for heating the thermal control medium; a first refrigerant/coolant heat exchanger functioning on the refrigerant side as a condenser/gas cooler; and a cooler downstream of the evaporator on the coolant side for selectively cooling the thermal control medium.
According to another aspect of the invention, there has been provided a motor vehicle having an improved air-conditioning system of the type described above.
According to still another aspect of the invention there has been provided a method for air-conditioning supply air for a passenger compartment in a motor vehicle that includes a refrigerant circuit comprising a compressor and an evaporator, wherein the evaporator comprises a second refrigerant/coolant heat exchanger, a coolant circuit comprising a heat-generating unit to be cooled and a heater for heating the thermal control medium, a first refrigerant/coolant heat exchanger functioning on the refrigerant side as a condenser/gas cooler, and a cooler downstream of the evaporator on the coolant side for selectively cooling the thermal control medium. The method comprises: selectively passing supply air through a cooler supplied with coolant that is cooled by the second refrigerant/coolant heat exchanger; and/or passing supply air downstream of the cooler through a heater supplied with coolant that is selectively heated in part by the first refrigerant/coolant heat exchanger.